A known system for treating exhaust gas passing through an exhaust system of a diesel engine comprises a diesel oxidation catalyst (DOC) associated with a diesel particulate filter (DPF). The combination of these two exhaust gas treatment devices promotes chemical reactions in exhaust gas and traps diesel particulate matter (DPM) as exhaust flows through the exhaust system from the engine, thereby preventing significant amounts of pollutants such as hydrocarbons, carbon monoxide, soot, SOF, and ash, from entering the atmosphere.
A DPF requires regeneration from time to time in order to maintain particulate trapping efficiency. Regeneration can occur naturally when conditions are favorable, but can also be forced, such as when the particulate loading reaches a level that is deemed excessive because it is beginning to affect engine performance and/or trapping efficiency. Consequently, an engine control system typically calculates particulate loading from time to time to determine if regeneration needs to be forced.
Regeneration is forced by creating conditions that will burn off trapped particulates. The creation of conditions for initiating and continuing regeneration typically involves elevating the temperature of exhaust gas entering the DPF to a suitably high temperature. Because a diesel engine typically runs relatively cool and lean, the post-injection of diesel fuel can be used as part of the strategy to elevate exhaust gas temperatures entering the DPF while still leaving excess oxygen for burning the trapped particulate matter.
A known strategy for determining the amount of trapped particulates in a DPF (i.e. calculating the particulate loading) is based on pressure-flow relationships. For a given exhaust flow rate through a DPF, the difference between DPF inlet pressure and DPF outlet pressure is an indication of particulate loading.
It is believed fair to say that there is a general recognition among those familiar with DPF regeneration that it is desirable that a regeneration strategy minimize the frequency at which regeneration is forced, but when doing so that the strategy not significantly delay regeneration when conditions disclose that regeneration is needed.
When an engine is operating in a steady state condition, i.e. at a substantially constant speed and a substantially constant load, pressure across and flow through a DPF are substantially constant. Sufficiently accurate measurements of those parameters enable a sufficiently accurate calculation of particulate loading to be made.
However, the manner in which motor vehicles are driven results in their engines not always operating in steady state condition. While steady state operation can occur during certain driving situations such as highway cruising, acceleration and deceleration create transients in engine operation. Consequently, periodic calculation of DPF particulate loading may occasionally be made during transient operating conditions.